Logarithmic amplifiers which are amplifiers having a logarithmic transfer characteristic, are used widely and one example of their use is in microwave instrumentation.
At present logarithmic amplifiers comprise a number of amplifiers having a linear characteristic connected sequentially via hybrid couplers. Each of the hybrid couplers is connected to a demodulator for demodulating an amplitude modulated signal and these are commonly known as video detectors. The outputs from all of the demodulators are then summed to provide the logarithmic output of the amplifier.
It is also known to use a single amplifier with a linear characteristic and then to apply the output of the linear amplifier to a demodulator for demodulating an amplitude modulated signal with the demodulator itself having a logarithmic transfer characteristic so that the output from the demodulator provides the logarithmic output from the amplifier.
Conventional linear amplifiers for operating at microwave frequencies usually include both a resonant input stage and a resonant output stage and accordingly they, and any resulting logarithmic amplifier including them, only has a limited bandwidth. This gives rise to difficulties when the resulting logarithmic amplifiers are required to handle short pulses for which a large bandwidth is required. Amplifiers capable of operating at microwave frequencies are expensive and previous attempts to improve their bandwidth have resulted in the further increase in their cost.